Light harvesting system employing microstructures for efficient light trapping

ABSTRACT

A light harvesting system employing a focusing array and a photoresponsive layer in which a plurality of microstructured areas is formed. Light received by the focusing array is injected transmissively into the photoresponsive layer through the microstructured areas. The injected light is retained in the photoresponsive layer by at least a total internal reflection and is propagated within the layer until it is substantially absorbed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional application Ser.No. 61/399,552 filed on Jul. 13, 2010, incorporated herein by referencein its entirety, and from U.S. provisional application Ser. No.61/402,061 filed on Aug. 21, 2010 incorporated herein by reference inits entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. §1.14.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device and method for harvestingradiant energy emanated by a distant radiant energy source,particularly, to collecting the sunlight and absorbing it by a lightsensitive material, medium or device. More particularly, the presentinvention relates to photovoltaic devices, solar cells and lightdetectors having light trapping microstructures or layers to improveabsorption of light within the light sensitive layer, and to a methodfor generating electricity from sunlight thereof.

2. Description of Background Art

Conventionally, photovoltaic solar cells or light detectors employ anactive photoresponsive layer that absorbs at least a portion of theelectromagnetic spectrum of the light and generates charge carriers dueto the photovoltaic effect. Since most photovoltaic materials absorbmuch more weakly in certain wavelengths than in the others, the activelayer has to have at least a minimum thickness to be able to absorb mostof the light to which the photovoltaic material is responsive.

One exemplary material suitable for converting light into electricity issilicon (Si). However, Si is an indirect bandgap semiconductor and ispoorly absorbing the long wavelength light. For the active layer madewith crystalline silicon the minimum thickness is typically between 200and 400 μm (micrometers). While Si is very abundant, stable andwell-suited for solar cell and light detector manufacturing, the cost ofthis thick layer of silicon is quite high which results in the high costof the devices.

Some other than crystalline silicon types of photovoltaic devices, suchas amorphous silicon thin-film cells, for example, allow for a muchsmaller thickness of the active layer. However, with certain wavelengthsbeing still absorbed very weakly, they usually require some form oflight trapping that would cause the incident light to pass through theactive layer multiple times thus improving the absorption. The lighttrapping is usually implemented in the prior art by texturing one ormore surfaces comprising the solar cell in order to scatter the incidentlight at different angles thus resulting in a longer average light paththrough the active layer. In case of a monocrystalline silicon cell,light scattering and trapping is conventionally provided bymicrostructures such as periodic or random pyramids on the front surfaceand a reflective or light scattering surface at the rear of the cell. Incase of an amorphous thin-film Si cell consisting of several layers, atransparent top conductor layer is often textured to scatter light andhence increase the light path through the active layer.

During light trapping, some scattered light can be trapped in the activelayer of the solar photovoltaic device by means of TIR which can evenallow for the multiple passage of a portion of solar rays through theactive layer thus resulting in a better absorption and sunlightconversion. However, the existing approaches for light trapping in thephotovoltaic devices cannot prevent for a substantial portion ofincident light to escape from the device without being absorbed. Forexample, in case of the front surface employing random pyramidalmicrostructures, a large portion of the escaping light is usually lostthrough this front surface due to the random nature of the secondaryinteractions of the light rays with the pyramids. Furthermore, up to 10percent or more light can be lost in conventional photovoltaic systemsdue to the reflection from front contacts or absorption by layers orsurfaces which produce no photovoltaic effect.

An additional problem encountered in photovoltaic devices is that mostphotovoltaic materials have a relatively large refractive index whichresults in poor light coupling efficiency due to the high reflectionlosses from the light receiving surface. The bulk crystalline Si, forexample, has the refractive index of 3.57 at 1,000 nm (nanometers) and5.59 at 400 nm which results in the Fresnel reflection of 32% to 49% ofthe incident light at 1,000 nm and 400 nm, respectively. Typically,these problems can be addressed by adding an antireflective layer to thelight receiving surface and/or surface microstructuring. However, theantireflective coating works efficiently only in a limited bandwidth andadds system cost and processing time, while the microstructures arestill somewhat inefficient for light coupling or otherwise are quiteexpensive to be used for mass production, considering that the entirearea of the photovoltaic device must be processed to cover it with thesemicrostructures.

These drawbacks of the prior art approaches and the loss of useful lightare hampering the utility of the photovoltaic devices. None of theprevious efforts provides an efficient solution for coupling andtrapping essentially all of the incident light and allowing it to passthrough the sufficient effective depth of photosensitive material orallow the light to interact with the active layer as many times asnecessary to cause the efficient light absorption in a controlledmanner. It is therefore an object of this invention to provide animproved light harvesting system employing a novel photovoltaicstructure with efficient light coupling and trapping thus minimizingenergy loss.

The present invention solves the above problems by providing a layeredstructure having correlated surface relief features or microstructuresthat allow for enhancing the light coupling efficiency, increasing thelight path through the photosensitive material and for trapping theincident light within the device by means of at least TIR. The lighttrapping causes multiple passage of the trapped light through thephotoresponsive (active) layer thus improving the light absorption andenergy conversion efficiency. Other objects and advantages of thisinvention will be apparent to those skilled in the art from thefollowing disclosure.

BRIEF SUMMARY OF THE INVENTION

The present invention solves a number of light harvesting problemswithin a compact system utilizing efficient light coupling and trappingmechanisms. Light is injected into a photoresponsive layer through lightinput ports using a focusing array and trapped within the layer so thatthe useful light absorption is substantially enhanced.

A light harvesting system employing microstructures for efficient lighttrapping and comprising a focusing array and a photoresponsive layer isdescribed. The focusing array comprises a plurality of light collectingelements distributed over a planar surface of the array. Each lightcollecting element is configured to collect light from a larger area andfocus the incident light onto a substantially smaller focal area. Thephotoresponsive layer comprises light input ports formed in its lightreceiving surface. Each light input port is disposed in energy receivingrelationship with respect to at least one light collecting element ofthe focusing array. More particularly, each light input port is disposedin a vicinity of the respective light collecting element and aligned(centered) with respect to its optical axis. Each light input port isconfigured to receive a focused light beam and communicate it into thephotoresponsive layer a sufficiently oblique angle so as to result in animproved light coupling and generally increased light path andabsorption in said layer compared to the case when light enters thephotoresponsive layer elsewhere through its light receiving surface.

The invention is amenable to being embodied in a number of ways,including but not limited to the following descriptions.

At least one embodiment of the invention is configured as a lightharvesting system comprising: (a) a photoresponsive layer configured tointernally absorb at least a portion of the light propagating throughits body; (b) a plurality of light input ports associated with a lightreceiving surface of said photoresponsive layer; and (c) a plurality oflight collecting elements within a planar focusing array configured forfocusing received light onto said light input ports. Each of theplurality of light input ports is configured to communicate the incidentlight into the photoresponsive layer at a sufficiently oblique angle soas to increase the optical path of the light rays through the designatedlayer. The device operates in response to the light received on theaperture of the focusing array being injected into the photoresponsivelayer and angularly redirected at generally oblique angles with respectto the prevailing plane of the light harvesting system.

In at least one implementation, the light harvesting system furthercomprises at least one electrical contact associated with thephotoresponsive layer. In at least one further implementation, theelectrical contact associated with the photoresponsive layer is madefrom an optically transparent material. In at least one implementation,the electrical contact associated with the photoresponsive layercomprises a reflective metallic material and is made in the form of agrid. In at least one implementation, the electrical contact comprises areflective metallic material and is made in the form of a thin sheet ora film. In at least one implementation, the electrical contact comprisesa plurality of electrical contact fingers disposed in spaces betweenadjacent pairs of light input ports and away from the light paths of thefocused beams formed by the light collecting elements. In at least oneimplementation, the photoresponsive layer comprises at least onephotovoltaic cell.

In alternative implementations, the planar focusing array and its lightcollecting elements can be configured in different ways. In at least oneimplementation, the focusing array comprises a lenticular lens array. Inat least one implementation, the focusing array comprises a point-focuslens array. In at least one implementation, the focusing array comprisespoint focus lenses which have a shape selected from the group consistingof round, rectangular, square, and hexagonal. In at least oneimplementation, each of the light collecting elements is selected fromthe group of optical elements consisting of imaging lenses, non-imaginglenses, spherical lenses, aspherical lenses, lens arrays, Fresnellenses, TIR lenses, gradient index lenses, diffraction lenses, mirrors,Fresnel mirrors, spherical mirrors, parabolic mirrors, mirror arrays,and trough mirrors.

In different implementations, the plurality of light input ports can bedifferently configured. In at least one implementation, each of theplurality of light input ports is disposed in a predetermined alignmentwith the plurality of light collecting elements. In at least oneimplementation, the oblique propagation angle within the photoresponsivelayer is so selected as to result in the propagation of at least asubstantial portion of light rays at sufficiently high angles, above thepredetermined critical angle for total internal reflection (TIR), withrespect to a surface normal to at least one surface of saidphotoresponsive layer. In at least one implementation, each of theplurality of light input ports comprises a refractive or reflective faceinclined at an angle with respect to the prevailing plane of thephotoresponsive layer. In at least one implementation, the plurality oflight input ports comprises a parallel array of elongated grooves. In atleast one implementation, each of the plurality of light input portscomprises at least one cavity. In at least one implementation, thecavity has a sufficiently high aspect ratio. In at least oneimplementation, each of the light input ports comprises at least onesurface relief feature selected from the group of elements consisting ofcavities, holes, extensions, bulges, prisms, prismatic grooves, cones,conical cavities, funnel-shaped cavities, surface texture, reflectivesurfaces, refractive surfaces, diffraction gratings, holograms, lightscattering elements, and so forth.

In further implementations, the light harvesting system is can beconfigured in various ways to enhance the light trapping in thephotoresponsive layer. In at least one implementation, the lightharvesting system further comprises an optical interface disposedbetween the photoresponsive layer and the focusing array andcharacterized by a drop in refractive index in the direction of lightpropagation from the photoresponsive layer toward the focusing array. Inat least one implementation, the light harvesting system furthercomprises means for promoting a total internal reflection in thephotoresponsive layer.

In at least one implementation, the focusing array and thephotoresponsive layer are adapted for being retained in a translated, areversed and/or a rotated orientation relative to each other towardadjusting the acceptance angle or for tracking the source of light. Inat least one further implementation, the light harvesting system cancomprise at least one tracking means for tracking a light source.

At least one embodiment of the invention is configured as a lightharvesting system having a layered structure and comprising: (a) aphotovoltaic layer disposed between a first and a second reflectivesurfaces; and (b) a light focusing layer having surface relief featureseach configured to focus the incident light. At least one of the firstand second reflective surfaces comprises light input ports disposed inenergy receiving relationship to the surface relief features andconfigured to inject incident light into the space between the first andsecond reflective surfaces so as to cause multiple passage of lightthrough the photovoltaic layer.

In at least one implementation, at least one of the first and secondreflective surfaces is configured for reflecting light by means of atleast TIR. In at least one implementation, the light harvesting systemis further comprising at least one cladding layer associated with atleast one of the first and second reflective surfaces. In at least oneimplementation, at least one of the first and second reflective surfacesis associated with a mirror layer comprising a metallic material. In atleast one implementation, at least one of the first and secondreflective surfaces is associated with a mirror layer comprising a Braggreflector. In at least one implementation, the light focusing layercomprises a lens array. In at least one implementation, the photovoltaiclayer comprises at least one photovoltaic cell. In at least oneimplementation, the photovoltaic layer is associated with at least onelight detector. In at least one implementation, each of the light inputports is formed by a microstructured area in at least one of the firstand second reflective surfaces.

The present invention provides a number of beneficial elements which canbe implemented either separately or in any desired combination withoutdeparting from the present teachings.

An element of the invention is a light harvesting system collectinglight over a given area and communicating the incident light into aphotoresponsive layer, such as a photovoltaic layer, with an enhancedlight coupling efficiency and with increasing the light path through thephotoresponsive layer.

Another element of the invention is a plurality of light collectingelements within a focusing array which collectively collect the incidentlight over a broad area and focus it into a plurality of small-aperturefocal areas.

Another element of the invention is the inclusion of distributed lightinput ports, such as areas having surface relief features,microstructures or texture, associated with a light receiving surface orthe interior of photoresponsive layer and disposing them in a lightreceiving relationship to the light collecting elements of the focusingarray.

Another element of the invention is the inclusion of one or morecladding layers or mirrored surfaces to enhance respectively the totalinternal reflection or the specular reflection of light within thephotoresponsive layer.

Another element of the invention is the use of electrical contacts whichcan additionally be provided with enhanced reflective properties topromote retaining the light within the photoresponsive layer.

Another element of the invention is the use of contact fingersassociated with the light receiving surface of the photovoltaic layerand disposed in the spaces between the pairs of adjacent light inputports and away from the light paths of the focused beams formed by thelight collecting elements.

Another element of the invention is the use of linear arrays of lightcollecting elements and/or light input ports which span the surface ofthe device, or a portion thereof. Alternatively, another element of theinvention is the use of point-focus arrays of light collecting elementsand/or light input ports as well as portions and combinations of linearand point-focus arrays thereof.

Another element of the invention is the use of a predetermined alignmentfor disposing the light collecting elements with respect to the lightinput ports and/or the electrical contacts.

Another element of the invention is the use of light input ports in theform of cavities, holes or other microstructures having a sufficientlyhigh aspect ratio.

Another element of the invention is the use of light input ports eachcomprising a refractive or reflective face inclined at an angle to thelight receiving surface of the photoresponsive layer for injecting atleast a substantial part of the incident light into the layer at anoblique angle.

Another element of the invention is the use of tracking means fortracking a light source.

A still further element of the invention is a light harvesting systemwhich can be utilized in a wide range of light collecting or lightsensing applications.

Further elements of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 and FIG. 2 are schematic diagrams and ray tracing forconventional photovoltaic systems.

FIG. 3 is a schematic view and ray tracing of a light harvesting systemin accordance with at least one embodiment of the present invention.

FIG. 4 is a perspective view of a focusing array according to at leastone embodiment of the present invention, showing the use of a planarlenticular lens array.

FIG. 5 is a perspective top view a focusing array according to at leastone embodiment of the present invention, showing the use of a planarlens array employing point focus lenses.

FIG. 6 is a perspective top view of a focusing array according to atleast one embodiment of the present invention, showing a differentarrangement and shapes of point focus lenses than were shown in FIG. 5.

FIG. 7 is a schematic view of a photovoltaic layer portion comprising anelongated V-shape groove, according to at least one embodiment of thepresent invention.

FIG. 8 is a schematic view of a photovoltaic layer portion comprising aV-shape groove which has a shorter length than the elongated grove shownin FIG. 7, according to at least one embodiment of the presentinvention.

FIG. 9 is a schematic view of a photovoltaic layer portion comprising acavity having a pyramidal shape, according to at least one embodiment ofthe present invention.

FIG. 10 is a schematic view of a photovoltaic layer portion comprising acavity having a conical shape, according to at least one embodiment ofthe present invention.

FIG. 11 is a schematic view of a portion of light harvesting systemaccording to at least one embodiment of the present invention.

FIG. 12 is a ray tracing of an incident ray being injected into aphotovoltaic layer at oblique angles by a V-shaped cavity formed in aphotovoltaic layer surface, according to at least one embodiment of thepresent invention.

FIG. 13 is a ray tracing of an incident ray being injected into aphotovoltaic layer by a funnel-shaped cavity or groove havingcurvilinear walls, according to at least one embodiment of the presentinvention.

FIG. 14 is a ray tracing of an incident ray being injected into aphotovoltaic layer through a blind hole formed in a photovoltaic layersurface, according to at least one embodiment of the present invention.

FIG. 15 is a ray tracing of an incident ray being injected into aphotovoltaic layer via a through hole, according to at least oneembodiment of the present invention.

FIG. 16 is a schematic ray tracing diagram of light trapping in aphotovoltaic layer, according to at least one embodiment of the presentinvention.

FIG. 17 is a schematic view and ray tracing of a light harvesting systemportion further incorporating a layer of transparent material between afocusing array and a photovoltaic layer, according to at least oneembodiment of the present invention.

FIG. 18 is a schematic perspective view and raytracing of a lightharvesting system portion in which light input ports are formed bytextures areas in a photovoltaic layer, according to at least oneembodiment of the present invention.

FIG. 19 is a schematic perspective view of a photovoltaic layer portionincorporating a plurality of prismatic surface relief features,according to at least one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the apparatus generally shown inthe preceding figures. It will be appreciated that the apparatus mayvary as to configuration and as to details of the parts withoutdeparting from the basic concepts as disclosed herein. Furthermore,elements represented in one embodiment as taught herein are applicablewithout limitation to other embodiments taught herein, and incombination with those embodiments and what is known in the art.

A wide range of applications exist for the present invention in relationto the collection of electromagnetic radiant energy, such as light, in abroad spectrum or any suitable spectral bands or domains. Therefore, forthe sake of simplicity of expression, without limiting generality ofthis invention, the term “light” will be used herein although thegeneral terms “electromagnetic energy”, “electromagnetic radiation”,“radiant energy” or exemplary terms like “visible light”, “infraredlight”, or “ultraviolet light” would also be appropriate.

In order to be able to compare and contrast the present invention withtypical photovoltaic structures, FIG. 1 and FIG. 2 are shown toillustrate the operation of a common crystalline silicon (Si) solar celltypically employing a photovoltaic layer 4 of mono- or polycrystallinesilicon with a p-n junction formed by joining n-type and p-type Si.Individual rays interacting with the photovoltaic structure areillustrated by line segments that, by way of example and not limitation,can represent the paths of individual photons of the incident light beamor otherwise represent possible light paths. In FIG. 1, a front contactis formed by contact fingers 24 attached to a front surface 32 ofphotovoltaic layer 4 and a back contact 16 is formed by a metallic layerattached to a back surface 34 of photovoltaic layer 4.

Referring further to FIG. 1, an incident light ray 100 emanated by adistant source and entering photovoltaic layer 4 is absorbed within thislayer with the generation of an electron-hole pair 50 due to the photoeffect. The front and back contacts of the cell collect the chargecarriers thus generating useful photocurrent in an external circuit (notshown). A ray 102 strikes a contact finger 24 and is absorbed orscattered without photocurrent generation. A ray 104 enteringphotovoltaic layer 4 is reflected from back contact 16 and exits backinto the environment without being absorbed and thus without producingthe photocurrent. Finally, a ray 106 striking front surface 32 ofphotovoltaic layer 4 is reflected from the surface without penetrationinto the photoactive layer and without useful absorption. Thus, ray 100represents a useful, absorbed photon or photons while rays 102, 104 and106 represent photons that are lost due to various optical lossmechanisms. It should be noted that the light rays shown in FIG. 1 donot necessarily represent all possible light paths or absorption andloss scenarios and are merely provided for the purpose of illustratingsome of the most common loss mechanisms.

In FIG. 2, a conventional pyramid microstructure of the photovoltaiccells is illustrated along with the typical light trapping mechanism(the front contacts, p-n structure and other layers customary tophotovoltaic devices, such as passivation, diffusion, antireflectivelayers, etc., are not shown here for clarity). When a ray 108 strikesthe surface of a pyramid formed in the front surface of photovoltaiclayer 4 and if it is reflected from the pyramid surface, it has anotherchance to enter layer 4 through the surface of an adjacent pyramid.Thus, the overall surface reflectivity is somewhat reduced compared tothe flat surface having no microstructures. Furthermore, since ray 106now enters layer 4 at a greater angle with respect to the surfacenormal, its effective path through the photoresponsive material isincreased which can result in better light absorption.

However, ray 108 bouncing from back contact 16 exits layer 4 the nexttime it interacts with the front surface. It will be appreciated bythose skilled in the art, that if the chance of light reflection fromthe front surface of layer 4 is further reduced and if the effectivelight path in layer 4 is further extended, the useful light absorptionwill increase thus resulting in the enhanced photocurrent generation. Inconventional devices, further reducing the front surface reflectivityrequires applying antireflective coatings or implementing more expensiveand less practical microstructuring methods for the entire surface. Theextension of the effective light path usually requires the increase inmaterial thickness thus resulting in greater material consumption whichcan be detrimental to the system cost effectiveness.

FIG. 3 illustrates, in a schematic cross-section, an embodiment of alight harvesting system 2 in accordance with at least one preferredembodiment of the present invention. Referring to FIG. 3, lightharvesting system 2 comprises a planar layered structure including afocusing array 6, a front contact exemplified by a grid of contactfingers 24, a photoresponsive layer exemplified by photovoltaic layer 4,and a back contact 16.

Focusing array 6, hereinafter referred to also as a lens array,comprises an array of imaging micro lenses 10. The lens array can bemade from of a sheet or planar plate or slab of a transparent materialby appropriate contouring its frontal surface and obtaining surfacerelief features approximating individual refractive lenses. Each lenshas a spherical or aspherical light input surface and is configured toreceive an impinging beam of light, as may emanate from a distant lightsource (not shown), and to focus this incident light in at least onedimension onto a focal area having a transversal size substantiallysmaller than the transversal size of the lens in the same dimension.

For the purpose of this invention and from the practical standpoint, theterms “focal area” or “focus” of an individual lens 10 of lens array 6should be understood broadly and generally refers to an area within theenvelope of the focused beam which said lens may form when exposed to anincident beam of light, where said area has a cross sectionsubstantially smaller than the cross section of respective lens 10.Accordingly, the focal area can include areas at a relatively smalldistance from the “ideal” focus of lens 10 and where the focused beamcan be convergent (before focus) or divergent (after focus).

It will be appreciated by those skilled in the art that focusing array6, when positioned with one side representing the entrance apertureperpendicular to the incident beam, will provide a plurality of foci onthe opposite side which will be spaced apart from each other inaccordance with the spacing of individual lenses in the lens array. Aplanar lens array formed by lenses 10 having the same focal length willform a plurality of foci in a common focal plane disposed at a distancefrom lenses 10.

In accordance with at least one embodiment of the present invention, thefocal length of each lens 10 in the lens array is selected toapproximate the thickness of the layer forming the array. Moreparticularly, when each micro lens in the lens array has a convexspherical shape with the radius of curvature R and the index ofrefraction of the transparent material forming the lens array is n, theapproximate lens array thickness T can be found from the followingexpression:

$T \approx {\frac{n\; R}{n - 1}.}$

Thus, focusing array 6 will form a plurality of foci in a plane locatedin a proximity of its back surface at a focal distance from lenses 10.

Focusing array 6 can have a linear, or linear-focus, geometry when it isbased on lenticular lenses 10 providing light focusing in one dimension.Alternatively, it can have a point-focus geometry when it is based onlenses 10 focusing the incident light in two dimensions. While across-section of system 2 is illustrated in FIG. 3, it should beunderstood that individual lenses 10 forming the focusing array 6 can beof any desired configuration which provides for concentration of thereceived light into a plurality of the focal areas, including but notlimited to lenticular, cylindrical, round, hexagonal, square,rectangular, linear-focus, point-focus, and can be packed with anydesired density across the entrance aperture of focusing array 6.

FIG. 4 illustrates an exemplary configuration of focusing array 6 as alenticular lens array employing cylindrical lenses 10. In a furtherexample, FIG. 5 illustrates a densely packed configuration of thefocusing array 6 formed by point-focus lenses 10 each having a squareaperture which allows to cover about 100% of the lens array with thelens area. In a yet further example illustrated in FIG. 6, point-focuslenses 10 have an alternative hexagonal shape which also provides forthe high packing density of the lenses in the lens array.

Lenses 10 can be fabricated using any conventional method such asreplication, molding, embossing, micro-machining, grinding, chemicaletching, beam etching and the like. The individual lenses 10 can beintegrated with focusing array 6 and preferably comprise the samematerial as the body of the array. Alternatively, lenses 10 can beformed on a transparent substrate sheet or film and fabricated of thesame or a different material than the substrate. Individual lenses 10can also be configured as separate pieces and attached to the substratesheet or plate. Suitable materials for making focusing array 6 and/orlenses 10 include but are not limited to optical glass, polymethylmethacrylate (PMMA), silicone, polycarbonate, polystyrene, polyolefin,any optically clear resin which is obtainable by polymerization andcuring of various compositions or other materials suitable for creatinga sufficiently transparent structure. The placement of lenses 10 infocusing array 6 can be according to any suitable spatial metric and byany desired means. For example, lenses 10 can be spaced apart,contacting each other or overlapping and can be positioned in anydesired pattern in the array.

Focusing array 6 can have one or more layers of anti-reflective coating(not shown) deposited on its frontal surface and/or the opposite surfacein order to reduce the Fresnel reflections at optical interfaces andimprove the light transmission of the array. Also, an anti-reflectivelayer can be embedded at any suitable part of system 2, e.g. between anyof its layers to further promote the transmissivity and overall systemefficiency. For these purposes, conventional anti-reflective coatingssuch as TiO₂ deposited by Atmospheric Pressure Chemical Vapor Deposition(APCVD) and Si₃N₄ deposited by Plasma Enhanced Chemical Vapor Deposition(PECVD) can be used, for example.

Additionally, any suitable encapsulation or adhesion promoting layerscan be incorporated into system 2. Particularly, EVA resin,fluoropolymer or optically clear silicone can be provided forencapsulating photovoltaic layer 4.

Photovoltaic layer 4 preferably has generally smooth front and backsurfaces so that the light scattering of light interacting with thesesurfaces is minimized. Photovoltaic layer 4 may include any suitablephotovoltaic element or structure that absorbs light and converts itinto charge carriers and/or electric current and can comprise one ormore photoabsorptive layers. More particularly, photovoltaic layer 4 mayincorporate any structures or design commonly found in solar cells ofphotovoltaic light detectors.

By way of example and not limitation, photovoltaic layer 4 can be madefrom crystalline silicon so that one side is p-type, dominated bypositive holes, and the other side is n-type, dominated by negativeelectrons. The free electrons and holes generated by light in thesilicon diffuse to the p/n junction junction formed by p-typecrystalline Si layer and n-type crystalline Si layer and then separateto produce an electric current.

In a further example, photovoltaic layer 4 may incorporate a thin filmphotovoltaic structure such as that found in amorphous silicon (a-Si)solar cells where a typical cell can employ a p-i-n design, in which amiddle intrinsic layer (i-type or undoped) is sandwiched between ap-layer and an n-layer. When photovoltaic layer is of a-Si cell type,the top layer can be p-type a-Si, the middle layer can be intrinsicsilicon layer, and the bottom layer is n-type a-Si. In this case, lightentering photovoltaic layer 4 is at least partially absorbed andgenerates free electrons and holes in the intrinsic region, which arethen separated by the electric field. Similarly, a CdTe photovoltaicstructure with an n-i-p design can be employed in which the top layercan be cadmium sulfide (CdS), the middle layer can be intrinsic CdTe,and the bottom layer can be zinc telluride (ZnTe). In a further example,photovoltaic layer 4 can be also designed as a copper indium galliumdiselenide (GIGS) structure.

In a yet further example, photovoltaic layer 4 can incorporate amulti-junction PV structure which can have the so-called cascade ortandem architecture and can be based on two or more individual cellswith different bandgaps stacked on top of one another. Photons in aspectral band that is not absorbed well by the first cell aretransmitted to the second cell, which then absorbs at least a portion ofthe remaining solar radiation while it can remain transparent to thelower-energy photons. This selective absorption process continuesthrough the entire stack of individual cells forming photovoltaic layer4 and being designed for the respective bandgaps so that more solarenergy spectrum is absorbed compared to a single-cell design of thephotovoltaic layer. A multi-junction cell can be made as a mechanicalstack of independently made solar cells or as a monolithic structure inwhich one complete solar cell is made first, and then the layers for thesecond cell are grown or deposited directly on the first.

In a yet further example, photovoltaic layer 4 can incorporate Si orother semiconductor photovoltaic materials in the form of particles,spheres, rods, clusters, quantum dots, nanodots, nanowires, etc.embedded into various transparent or semi-transparent materials, such asfilms of silicon oxide and silicon nitride abound. Such embeddedphotovoltaic features can be evenly or unevenly distributed with aprescribed concentration in the bulk of photovoltaic layer 4 and canallow for efficient conversion of light into electricity provided theoptical path of rays propagating in said photovoltaic layer issufficiently long for the full light absorption.

Photovoltaic layer 4 can be associated with ohmic metallic ornon-metallic contacts or electrodes which in turn can be connected to anexternal load. These contacts are exemplified by front contact fingers24 and back contact 16 in FIG. 3. The charge carrier separation inphotovoltaic layer 4 can be done by any conventional means, for example,using drift of carriers driven by an electrostatic field establishedacross the layer or using diffusion of carriers from zones of highcarrier concentration to zones of low carrier concentration. Either orboth front and back surfaces of photovoltaic layer 4 can have a fullarea metal or non-metal contact or a grid-like metal contact made up offine “fingers” and larger “busbars” which can be, for example,screen-printed onto the respective surface using a silver or aluminumpaste. If metal screen printing is used the contacts can be fired atseveral hundred degrees Celsius to form good ohmic contact with thematerial of photovoltaic layer 4 and can further be electro-plated tofurther enhance the device efficiency.

Back contact 16 can also be customarily covered with a suitableprotective backsheet material (not shown) for protecting it andphotovoltaic layer 4 from the environment. The backsheet can alsoinclude any suitable layers or coatings such as radiation protectivecoatings or films, scratch and stain resistant coatings, and the like.An illustrative example of the protective backsheet is Polyvinylfluoride (PVF), a fluoropolymer which is commercially available as afilm from DuPont and is sold under the Tedlar® brand.

According to at least one embodiment of the present invention, thephotoabsorptive layer, or layers, if more than one, that formphotovoltaic layer 4 can be made relatively thin in order to reduce theintake of expensive semiconductor materials. The photoabsorptive layercan be made so thin that it absorbs only a small portion of the incidentlight in a single path. For example, 10% or less incident sunlight canbe absorbed when the light passes through photovoltaic layer 4 in eitherdirection at normal incidence. The rest of the light, however, can beabsorbed in system 2 through multiple passages of light throughphotovoltaic layer 4 as well as through increasing the light paththrough the photoabsorptive layer(s) by means of communicating the lightoblique angles and/or by means of light trapping.

Referring further to FIG. 3, photovoltaic layer 4 is further configuredto have light input ports 14 formed in its front surface 32. Accordingto at least one embodiment of the present invention, each light inputports 14 can comprise at least one refractive surface or face formed bymicrostructuring surface 32 and disposed at an angle to surface 32.Light input ports 14 can include any suitable surface reliefmicro-structures such as interruptions, recesses, holes, cavities,micro-extensions or bulges in otherwise smooth surface 32 which promotelight coupling into photovoltaic layer 4. A variety of methods can beutilized for creating such surface relief micro-structures. By way ofexample, cavities of various shapes and profiles can be fabricated usinga technique for direct material removal including drilling, mechanicalscribing, laser scribing, micromachining, chemical etching, ion beametching, reactive ion etching, plasma etching or any other known methodsuitable thereof for surface micro structuring. In addition, the wallsof the formed cavities can be optionally polished, as desired, to obtainany suitable level of polished smooth surface.

Light input ports 14 have the same number as the number of lenses 10 infocusing array 6 disposed on top of photovoltaic layer 4. Light inputports 14 are also distributed over surface 32 in accordance with thedistribution of lenses 10 in the lens array so that there is a pairwiseoptical relationship between each lens 10 and the corresponding lightinput port 14. It is also preferred that lenses 10 and light input ports14 are distributed so that contact fingers 24 are disposed between theoptical axes of lenses 10 and away from the optical paths in order toallow the focused light to enter photovoltaic layer 4 through lightinput ports and minimize light interception by contact fingers. In otherwords, when contact fingers 24 are disposed between the optical centersof lenses 10, each lens 10 effectively steers the focused light awayfrom the contact fingers and eliminates light shadowing by the frontelectrical contacts thus further enhancing the light coupling and systemefficiency.

When focusing array 6 is formed by a lenticular lens array, each lightinput port 14 can extend parallel to the respective lens 10 and alongits entire length. In a cross-section, such as that illustrated in FIG.3, the receiving aperture of each light input port 14 can be centeredwith respect to the optical axis of the matching lens 10.

It is important that at least the transversal size of each light inputport 14 is substantially smaller than the distance between the adjacentlight input ports so that the cumulative area occupied by light inputports 14 on surface 32 is substantially less than the area of saidsurface. Yet, the receiving aperture of each light input port 14 shouldbe sufficient in order to intercept at least a substantial portion ofthe light focused by the respective lens 10. It is generally preferredthat light input ports occupy no more than approximately 10% of surface32.

According to at least one embodiment of the invention, the transversalsize of each light input port 14 is selected to approximate that of thefocal area of the respective lens 10. In this case and when, forexample, focusing array 6 is formed by densely packed cylindrical lenseseach having the focal area which is about 20 times smaller than theaperture of the lens, light input ports 14 will cumulatively occupyapproximately 5% of the area of surface 32. In a further example, ifeach lens 10 has a point focus with 400 times geometrical concentration(the geometrical concentration being defined as the ratio between theaperture of the lens and the focal area in a plane perpendicular to thelens's optical axis), the cumulative area of light input ports 14 willmake approximately 0.25% of the area of surface 32.

Referring yet further to FIG. 3, each input port 14 is formed by arecessed surface relief feature having a V-shaped profile in across-section and at least one open refractive face inclined at an anglewith respect to surface 32. When light input port 14 has multiple faces,at least one such face can be mirrored for specular reflectivity.

FIGS. 7 through 10 illustrate various exemplary configurations of lightinput port 14 which can be represented by a V-shaped cross-section shownin FIG. 3. Particularly, FIG. 7 shows an elongated V-groove which canextend all the way along surface 32 or its substantial portion. FIG. 8shows a shorter V-groove which can still have an elongated shape or itcan have same or similar longitudinal and transversal dimensions. FIG. 9shows a pyramidal shape of the cavity representing light input port 14.FIG. 10 shows a cone-shape cavity forming light input port 14. Lightinput ports 14 can also be formed by cavities having the shape of afunnel with curvilinear walls which can operate similarly to the conicalshape.

In at least some embodiments of the present invention, light input ports14 are formed by high aspect ratio cavities or recesses in surface 32.In the context of describing a surface relief feature or microstructureelement such as a surface cavity or recess, the term “high aspect ratio”is meant to mean a geometric configuration of the microstructureelement, in a cross-section, where the height or depth of themicrostructure element is approximately equal or greater than its baseat the surface. This term also includes the case when the height of themicrostructure element is much greater than the base thus correspondingto a relatively deep cavity or recess with almost vertical walls. Eachcavity or recess can have at least one face which can be perpendicularto or inclined at an angle to surface 32.

According to an embodiment of the present invention illustrated in FIG.3, each individual light input port 14 can be viewed as any suitablelocalized interruption or alteration of the otherwise smooth surface 32that alters the optical interface properties of the surface in such away that a fan of rays entering photovoltaic layer 4 through any lightinput port 14 will be communicated a different angular distribution inat least one dimension within photovoltaic layer 4 compared to the casewhen the same fan of rays enters photovoltaic layer 4 through the smoothportion of surface 32 elsewhere. In an aspect of this invention, lightinput ports 14 suppress the normal Snell's law refraction generallycharacterizing the broad surface 32. It will be appreciated that lightinput ports 14 alter the surface properties only within their activeapertures while the rest of the surface area remains largely unaffected.

In at least one embodiment of FIG. 3, when focusing array 6 andphotovoltaic layer 4 are sandwiched together, the cavities of lightinput ports 14 can remain filled by atmospheric-pressure air trappedduring the assembly. Alternatively, system 2 can be assembled undervacuum and focusing array 6 and photovoltaic layer 4 can be sealedtogether so that the cavities of light input ports 14 can containlow-pressure air or vacuum.

In operation, referring yet further to FIG. 3, incident light enteringsystem 2 is captured and injected into photovoltaic layer 4 at obliqueangles which allows for increasing the light path inside thephotoresponsive layer(s) and for trapping the light between the parallelsurfaces of photovoltaic layer 4, as illustrated by the paths ofexemplary rays 110 and 112. Ray 110 is collected by one of the lenses 10of focusing array 6 and directed to light input port 14 disposed in thevicinity of the focus of the lens 10 communicating an initial bend angleto ray 110. Light input port 14 communicates a further bend angle to ray110 resulting in an oblique incidence of the ray into photovoltaic layer4. Since light input port 14 is structured as a cavity, the lightcoupling of ray 110 is improved compared to the case when light entersphotovoltaic layer through its planar surface and when the incident raycan be reflected from the surface of high-refractive-index photovoltaiclayer 4 due to the Fresnel reflection. Furthermore, the light path ofray 110 is increased compared to the case when light enters photovoltaiclayer through its smooth surface.

Obviously, some rays propagating in photovoltaic layer can strike one ofthe light input ports 14 and can be scattered, reflected or absorbed asa result. Such secondary interactions with light input ports 14 can alsoresult in the respective ray exiting photovoltaic layer 4 without beingfully absorbed. However, since the size of microstructures forming lightinput ports 14 and distributed through surface 32 is relatively small,the chance of ray 110 encountering another light input port anddecoupling from photovoltaic layer 4 is also reduced compared to thecase when the entire surface 32 is covered with microstructures (as in aconventional monocrystalline Si cell, for example). It will beappreciated by those skilled in the art that the greater theconcentration achieved by lenses 10 and the smaller the respective focalspots and thus the smaller light input ports 14 are, the lesser lightlosses will be and the greater light absorption efficiency can beachieved.

Similarly, ray 112 is injected into photovoltaic layer 4 at an obliqueangle by another matching pair of lens 10 and light input port 14.Accordingly, ray 112 can also propagate through a greater effectivethickness of the photoresponsive material due to off-normal propagationand due to bouncing from parallel opposing surfaces 32 and 34 until itis absorbed.

While the pairs of lenses 10 and light input ports 14 work best forinjecting a direct normal-incidence beam received by the entranceaperture of focusing array 6 into photovoltaic layer 4 at advantageouslyoblique angles, system 2 can harvest off-axis rays as well asillustrated further in FIG. 3. An exemplary off-axis ray 160 strikingone of lenses 10 passes through the transparent body of focusing array 6and enters photovoltaic layer 4. Ray 160 can represent any applicableoff-axis light such as, for example, stray light, diffuse backgroundlight, or direct beam when system 2 is not pointed directly at the lightsource. In the illustrated case, the angle of incidence of ray 160 ontofocusing array is greater than the acceptance angle of lens 10 so thatthis off-axis ray can miss the corresponding light input port 14.Therefore, ray 160 enters photovoltaic layer 4 through a smooth portionof surface 32. While the efficiency of light coupling and trapping ofray 160 can be considerably less than that of rays 110 and 112, this raymay nevertheless also propagate through some thickness of photovoltaiclayer and therefore also have a chance of being subsequently absorbed.

FIG. 11 illustrates the operation of system 2 shown in FIG. 3 in afurther aspect. In FIG. 11, an incident ray 120 entering focusing array6 at an normal incidence with respect to the prevailing plane of system2 strikes an individual lens 10 and is directed to the respective focusof the lens where it enters matching light input port 14 formed insurface 32 of photovoltaic layer 4. Ray 120 refracts at a refractiveface of the V-shaped cavity forming the light input port 14 and isinjected into photovoltaic layer 4 at an oblique angle. Ray 120propagates in photovoltaic layer until it is absorbed and converted intoelectron-hole pair 50 due to the photo effect.

Due to the combined effect of two-stage bending, the incident ray bylens 10 and light input port 14, ray 120 is communicated a greaterpropagation angle with respect to the surface normal of photovoltaiclayer 4 as compared to the case when the same ray would enterphotovoltaic layer 4 of conventional devices such as those illustratedin FIG. 1 and FIG. 2. Therefore, it can propagate through an effectivelythicker layer of photoresponsive material before reaching any surface ofphotovoltaic layer 4 and thus has a greater chance of being absorbed ina single pass.

Furthermore, even if ray 120 is not absorbed in a single pass, it canbounce from surface 34 by means of a specular reflection or scattering,in which case it can pass through photovoltaic layer 4 at least one moretime which increases the chance of absorption with photocurrentgeneration. This is illustrated by a ray 122 which enters photovoltaiclayer 4 at a similar angle as ray 120 but is not absorbed in the singlelayer passage. Ray 122 reaches surface 34, reflects from the surfaceback into photovoltaic layer 4 and is absorbed with the generation ofanother electron-hole pair 52.

According to at least one embodiment of this invention, sufficientspecular reflectivity of surface 34 can be provided by metallic backcontact 16 so as to result in minimum reflection losses. By way ofexample and not limitation, if photovoltaic layer 4 comprises acrystalline Si cell, back contact 16 can be conventionally made fromaluminum by means of sputtering, sintering, plating or by applying analuminum film or foil onto surface 34. Aluminum back contact 16 can besubsequently fired at high temperature which will normally provide p+doping for a Si-type cell and improve the photovoltaic performance ofthe device. Alternatively, contact 16 can be made by depositing a silverlayer which will provide both good reflectance of surface 34 and backcontact conductivity.

Obviously, when the light beam focused by lens 10 enters the cavity oflight input port 14, at least some rays can undergo Fresnel reflectionfrom the surface of the cavity, especially considering the highrefractive index of photovoltaic layer 4. Therefore, the shape of thecavity can be selected to provide for improved coupling of the reflectedlight. By way of example and not limitation, when light input port 14 isformed by a V-shaped cavity, the cavity can be provided with the aspectratio high enough to prevent escaping the reflected light fromphotovoltaic layer without having at least a second interaction with thecavity wall(s). Furthermore, the aspect ratio can be so selected as toallow for multiple Fresnel reflections from the cavity walls thussubstantially enhancing the light coupling efficiency into photovoltaiclayer 4. Every time the light ray interacts with the surface of thecavity it can enter photovoltaic layer 4 with some refraction whileobtaining a greater angle with respect to photovoltaic layer 4 surfacenormal. The refracted ray can thus further propagate in photovoltaiclayer 4 at a sufficiently high angle with respect to a surface normal.Since the cumulative base area and the cross-section area of light inputports is sufficiently small compared to the frontal area a cross-sectionof photovoltaic layer 4, the probability of the propagating ray exitingthrough another light input port 14 is relatively low. Therefore, evenif the propagation path of the ray is not parallel to the longitudinaldimensions of photovoltaic layer 4 and said ray can strike eithersurface 32 or 34 multiple times, it will strike mostly the surfaceportions free of any microstructures and can thus be reflected back intophotovoltaic layer 4 maintaining the same incidence angle. The ray willthereby become trapped and its propagation in photovoltaic layer 4 cancontinue, which may include additional bounces from surfaces 32 and 34until the ray energy is absorbed and converted to electric current.

FIG. 12 illustrates light coupling through a V-shaped cavity in moredetail. A ray 126 of FIG. 12 approaches layer 4 at a near-normal angle,after passing through lens 10 of lens array 6 (not shown), and entersthe entrance aperture of light input port 14 where it strikes aninclined face 26 of the cavity at a point 72. Upon entering intophotovoltaic layer 4, ray 126 can undergo refraction with a bend angledefined from the relationship n₁ sin θ₁=n₂ sin θ₂ (Snell's law), whereθ₁ is the angle of incidence, θ₂ the angle of refraction, and n₁ and n₂are the refractive indices of the medium inside the cavity of lightinput port 14 and the material of photovoltaic layer 4, respectively.Angles θ₁ and θ₂ are counted off a normal to face 26 at point 72. By wayof example, if photovoltaic layer 4 is made of crystalline Si having therefractive index n₂ of approximately 3.5 and the medium inside grove 14is air or vacuum with the refractive index n₁ of about 1, the refractionangle can be determined as follows: θ₂=arcsin(0.29 sin θ₁).

When ray 126 undergoes refraction at point 72, it can further propagatein the medium of photovoltaic layer 4 (the respective ray segment isdenoted as 156). When photovoltaic layer 4 comprises material withsufficiently high refractive index, the larger bend angles can result inincreasing the optical path within the photovoltaic layer before itreaches surface 34. Segment 156 or ray 126 can undergo multiplereflections from surfaces 34 and 32 passing photovoltaic layer 4multiple times till it is absorbed or, when the bend angle is largeenough, ray segment 156 may even never reach either surface 32 or 34before being essentially absorbed and converted to electricity.

Referring further to FIG. 12, ray 126 can also undergo Fresnelreflection from face 26 at point 72. Alternatively, face 26 may beprovided with a mirrored surface in which case ray 126 will undergo aspecular reflection. The corresponding reflected ray segment is denotedas 158. Although the Fresnel type of reflection typically results in asmaller portion of incident beam energy going into the reflected rays, agreater refractive index of the photovoltaic layer medium and a greaterincidence angle can result in a sizable portion of incident ray energyreflecting from the cavity surface. Accordingly, ray segment 158 willcross the volume of the cavity and enter photovoltaic layer 4 through anopposing face 28 of the cavity, even at a greater angle with respect toa normal to the layer's planar surface than segment 156 thus increasingthe ray path in it and enhancing the absorption efficiency. Similarly,segment 158 or ray 126 can propagate between surfaces 34 and 32 passingthrough the effective thickness of photovoltaic layer 4 multiple timestill it is absorbed.

Although a near-normal incidence of ray 126 is illustrated in FIG. 12,any other incidence angle within a predetermined range of acceptanceangles can be efficiently injected into photovoltaic layer 4 throughlight input ports 14 formed by a V-shaped cavity. Furthermore theV-shaped cavity forming light input port 14 is not limited to havestraight walls as illustrated in FIG. 12. According to at least oneembodiment of the present invention, the cavity can also havecurvilinear funnel-shaped walls which can naturally result, for example,from the process of formation of light input ports 14 by means ofmaterial removal using laser ablation, etching or the like.

FIG. 13 shows an example of light input ports 14 formed by a funnelshaped cavity with curved or curvilinear walls and also shows an exampleof the off-normal incidence of ray 126 onto the entrance aperture of theexemplary light input port 14. Accordingly, ray segments 156 and 158represent possible light paths of ray 126 after interacting with face 26of light input port 14 at point 72. In either illustrated case of FIG.13, ray 126 enters photovoltaic layer 4 and becomes trapped in it withan increased probability of being absorbed due to the extended lightpath through the photoactive material.

FIG. 14 illustrates an embodiment of light harvesting system 2 in whichan individual light input port 14 is formed in photovoltaic layer 4 by around blind hole having essentially vertical walls. This hole can beassociated with the respective lens 10 of focusing array 6 (not shown)having a point-focus configuration. In operation, ray 126 enters theentrance aperture of the blind hole forming light input port 14 andstrikes its vertical wall 30 at point 76. Ray 126 can directly refractinto photovoltaic layer 4, as indicated by ray segment 156, or it canenter photovoltaic layer 4 after reflecting once from wall 30, asindicated by ray segment 158. In either case, as a matter of optics,when ray 156 enters photovoltaic layer 4, it has a greater angle withrespect to layer 4 surface normal than the incidence angle upon theentrance onto the receiving aperture of light input port 14, thusincreasing the length of the optical path in layer 4 and enhancingabsorption. Furthermore when surfaces 32 and 34 have reflectiveproperties, ray 26 will become trapped in the photovoltaic layer 4 andcan travel within it until fully absorbed.

Similarly, in FIG. 15, a further example of light input port 14 is shownwhere it is formed by a through hole in photovoltaic layer 4.Accordingly, ray 126 entering the through hole is trapped and absorbedwithin photovoltaic layer 4. The holes representing light input ports 14can be made in layer 4 by means of laser ablation, wet or dry etching,drilling or the like processes. Wet chemical etching can also be usedfor the subsequent processing of light input ports 14 after they areformed to enhance surface smoothness and/or for drilling damage or slagremoval. While the holes forming light input ports 14 are shown havingvertical walls in FIG. 14 and FIG. 15, it should be understood that thisinvention is not limited to this and can be applied to the case when theholes are made at an angle to the surface normal of photovoltaic layer.Furthermore, some processes of making the holes may leave non-verticalwalls so that an individual hole can have varying diameter along itslongitudinal axis. By way of example, if an individual hole is made bylaser drilling it can have a slightly conical shape or a curvilinearfunnel shape. It should be understood that the embodiments shown in FIG.14 and FIG. 15 in a cross-section are not limited to the round holes andcan include the cases when the respective light input ports 14 areformed by deep linear trenches or grooves having vertical ornear-vertical walls and which can be illustrated by essentially the samecross-sections.

FIG. 16 further illustrates light trapping in photovoltaic layer 4 ofsystem 2. In FIG. 16, rays 130, 132, 134, and 136 represent various raysinjected elsewhere into photovoltaic layer 4 by respective pairs oflenses 10 and light input ports 14 and propagating in the photovoltaiclayer 4 after having been reflected at least once from surface 34.Particularly, ray 130 reaches surface 32 where it encounters an opticalinterface between photovoltaic layer 4 generally having a higherrefractive index n₁ and the outside medium generally having a lowerrefractive index n₂.

As a matter of optics, when the angle of incidence of ray 130 ontosurface 32 is greater than arcsin n₂/n₁, ray 130 will undergo avirtually lossless total internal reflection (TIR) back intophotovoltaic layer 4. The TIR condition can be relatively easy achievedin system 2 for most rays propagating in photovoltaic layer 4,considering the high refractive index of most photovoltaic materialssuch as Si (n₁≧3.5) and the low refractive index of the ambient air(n₂≈1), in which case the critical TIR angle will be approximately 16.6°or less, depending on the wavelength. The critical angle at which TIRoccurs will define the escape cone in relation to the angulardistribution of rays impinging onto surface 32 internally from theinside of photovoltaic layer 4 so that when an incident ray is outsideof this cone, it will become trapped in photovoltaic layer 4.

Even when a layer of conventional encapsulating material is used as anintermediate layer between focusing array 6 and photovoltaic layer 4,such as EVA (n₂≈1.48) or optically clear silicone (n₂≈1.4), the criticalTIR angles at the interface with bulk silicon will be approximately 25°and 23.6°, respectively. Accordingly, the optical configuration oflenses 10 and light input ports 14 can be optimized to providesufficient bend angles for the entire incident beam so that the TIRcondition is met for all rays propagating in photovoltaic layer 4. Forthis purpose, each surface relief feature 14 should be configured toreceive the focused light beam from the respective feature 10 and injectsaid focused beam into photovoltaic layer with a new angular pattern sothat most of the rays in the injected beam will make generally greaterangles with respect to a normal to the prevailing plane of system 2 thanthe half-angle of the escape cone.

The escape cone can be further minimized by selecting low-n materialssuch as fluoropolymers or certain silicones having the refractiveindices in the 1.29-1.36 range or by providing a cladding layer madefrom any of these materials. In a further alternative, surface 32 can beprovided with specular reflective propertied by means of metallizationof depositing other highly reflective layers such as, for example,distributed Bragg reflector (DBR).

Similarly, ray 132 of FIG. 16 is shown undergoing TIR or specularreflection from surface 32 and is further shown encountering a wall ofthe V-shaped cavity of light input port 14. Since the medium in thecavity is air or vacuum with n₂≈1, ray 132 undergoes a further TIR fromthe structure of light input port 14 and still remains trapped inphotovoltaic layer 4.

Ray 134 strikes a lower portion of another V-shaped cavity forming lightinput port 14 of FIG. 16. As it is shown, ray 134 can temporarily exitthe bulk material of photovoltaic layer 4 with refraction and thenfurther re-enter it through the same cavity thus remaining trapped insystem 2.

Referring further to FIG. 16, ray 136 strikes one of the contact fingers24 from its back. In a non-limiting example of crystalline Siphotovoltaic cells, the contact fingers can commonly be made byscreen-printing onto the front surface using a silver paste which isreflective in a broad spectrum of light. Ray 136 is subsequentlyreflected from contact finger 24 back into photovoltaic layer 4 thusalso remaining trapped in system 2. Since the specular type ofreflection can be more advantageous than the diffuse one in theillustrated case, the specular reflectivity of the front contacts can beenhanced by depositing a mirror-finish silver thin film on surface 32 inthe respective areas before screen printing the contact fingers 24.

In FIG. 17, a transparent buffer layer 30 is shown provided betweenfocusing array 6 and photovoltaic layer 4. This buffer layer can serve acongruence of goals. It can provide encapsulation for the photovoltaiclayer 4 and its contacts from the ambient air and moisture. Layer 30 canalso act as an adhesive layer between focusing array 6 and photovoltaiclayer 4 and assist in forming a monolithic structure of light harvestingsystem 2. Additionally, it can serve as low refractive index claddingfor photovoltaic layer 4 and promote light trapping by means of TIR.

Referring to FIG. 17, light rays 120 and 122 received and focused bylenses 10 enter photovoltaic layer 4 through the respective light inputports 14 and propagate in photovoltaic layer 4 as in a waveguide bybouncing from surfaces 32 and 34 until absorbed. Similarly to FIG. 16,the reflection of light from surface 32 by means of TIR is illustratedby ray 130 in FIG. 17.

Although the preceding embodiments described light input ports whichwere formed by individual cavities in surface 32, the present inventioncan be implemented with each light input port 14 formed by a texturedarea having multiple surface relief features or microstructural elementsthat can enhance light coupling into photovoltaic layer 4 and minimizereflection from surface 32. Any suitable surface texture that allows theincident light to pass through surface 32 with a reduced reflection andgreater off-normal propagation angles in photovoltaic layer 4 comparedto light entrance through the smooth portions of surface 32 can be usedfor making light input ports 14. By way of example and not limitation,each light input port 14 can include random or ordered arrays ofhigh-aspect-ratio pyramids or inverted pyramids, nano-rods, nano-wires,close positioned micro-cavities, micro-holes, micro-extensions orbulges, etc.

FIG. 18 shows an illustrative example of system 2, where each lightinput port 14 associated with surface 32 is formed by an array ofV-shaped micro cavities or grooves having the sizes substantiallysmaller than the active aperture of the respective light input port. Inoperation, ray 110 is focused by lens 10 onto the respective light inputport 14 which inject said ray into photovoltaic layer 4 at an obliqueangle so that it can be trapped and absorbed between surfaces 32 and 34.

In FIG. 19, a portion of photovoltaic layer 4 is shown where light inputport 14 is positioned between contact fingers and is formed by atextured surface area comprising sharp peaks and valleys of randompyramids. This exemplary configuration of light input port 14 alsopromotes light coupling with reduced surface reflection. The texturedarea covers only a relatively small portion of surface 32 and thereforedoes not substantially affect the overall surface smoothness and opticalproperties.

It will be appreciated that diffusers or a random light scatteringmicrostructure or surface texture can also be used for forming lightinput ports 14 in which case the light will be more randomly distributedacross a range of angles in photovoltaic layer 4. A further example ofuseful feature 14 can be a matte-finish textured area in surface 32having the aperture approximating the dimensions of the focal area ofthe respective lens 10. When light-scattering surface texture is usedfor forming light input ports 14, a portion of light injected intophotovoltaic layer 4 at near-normal angles may escape from the system.However, due to the usually high refractive index of the material ofphotovoltaic layer 4 and the resulting small TIR escape cone, asubstantial portion of the incident light can still be trapped in amanner described above thus resulting in a generally longer optical pathof the rays within the photoresponsive layer and improved photocurrentgeneration. The light injection of incident light into photovoltaiclayer 4 at advantageously high angles with respect to the surface canalso be provided using diffraction gratings or holograms which canadditionally be tuned for a specific spectral band. For example, whenthe photoresponsive layer of photovoltaic layer 4 comprises crystallinesilicon which has reduced absorption in the infrared, light input ports14 can be tuned to provided greater bend angles for the wavelengths inthe range of 800-1100 nanometers.

Although the preceding embodiments described font electrical contact ofphotovoltaic layer 4 which was configured as metallic contact fingers 24attached to surface 32, the present invention can be implemented usingother types of front and back electrical contacts. By way of example andnot limitation, a transparent electro-conducting layer can be attachedto surface 32 instead or in addition to contact fingers 24. Suchtransparent front contact can be made from any conventional transparentconducting material. Particularly, transparent conducting films (TCFs)conventionally used for photovoltaic applications can be employed. TCFcan be fabricated from inorganic and/or organic materials. An example ofinorganic films is a layer of transparent conducting oxide (TCO).Suitable materials for the TCO include, but are not limited to,aluminum-doped zinc oxide (AZO), boron-doped zinc oxide, fluorine dopedtin oxide (FTO), indium tin oxide (ITO), indium molybdenum oxide (IMO),indium zinc oxide (IZO) and tantalum oxide. The TCO layer can bedeposited onto surface 32 by any suitable process, such as chemicalvapor deposition (CVD) or physical vapor deposition (PVD). The thicknessof the transparent electro-conducting layer can be fairly small comparedto the thickness of photovoltaic layer 4 and can be typically up toabout a few thousand nanometers. Furthermore, back contact 16 can alsobe implemented as a transparent electro-conducting and made as describedabove.

The foregoing embodiments are described upon the case when back contact16 is applied to surface 34 and is also used as a reflective surface fortrapping the light in photovoltaic layer 4. However, this invention isnot limited to this but can also be applied to the case when anintermediate reflective layer is provided between photovoltaic layer 4and back contact 16 to enhance the internal reflection at surface 34. Byway of example and not limitation, the intermediate reflective layer mayinclude a Bragg reflector formed by a stack of alternating porous Silayers of high and low porosity defined by the quarter-wavelengthprinciple. The porous Si layers can be electrochemically etched into ahighly doped Si substrate to create constructive interference for theefficient reflection of the incident light for a pre-selected wavelengthrange and thus enhance the reflectivity of surface 34 and light trappingin photovoltaic layer 4.

The foregoing embodiments are described upon the case when focusingarray 6 was configured as an array of imaging lenses. However, thisinvention is not limited to this but can also be applied to the casewhen focusing array 6 may comprise any desired optical structures orsurface relief features distributed over its frontal surface and adaptedfor collecting or concentrating the impinging light. Any known lightfocusing structure which collects the energy from a larger area andfocuses it to a smaller focal area can be used to form the individualfocusing elements of focusing array 6. By way of example and notlimitation, focusing elements of the array can be selected from thegroup of optical elements consisting essentially of spherical oraspherical refractive lenses, Fresnel lenses, TIR lenses, gradient indexlenses, diffraction lenses, lens arrays, mirrors, Fresnel mirrors,mirror arrays, trough mirrors, and the like.

It should be understood that while the preceding embodiments wereillustrated by depicting only a few exemplary rays, system 2 can operatewith any number of rays within a selected acceptance angle and in adesired spectral range of the incident light. It should also beunderstood that focusing array 6 can collect a beam of light emanated bya distant source, focus the beam and communicate it to light input ports14. Accordingly, light input ports 14 can be configured to receive aconvergent or divergent beam of focused light having a predeterminedangular spread and redirect the beam into photovoltaic layer 4 at asufficient bend angle so that each ray of the redirected beam becomestrapped in photovoltaic layer 4 and further propagates in it by meanstransmission and reflection until absorbed. It will be appreciated bythose skilled in the art that system 2 can be used to effectivelycapture a quasi-parallel beam of monochromatic or broad-spectrumelectromagnetic energy, trap and guide it through its photovoltaicelement or layer so that substantially all of the beam can be absorbedand converted into the electric current using a much thinner layer ofphotoabsorptive material than in conventional devices.

Furthermore, it should be understood that, as illustrated by foregoingembodiments, the surfaces confining the captured light withinphotovoltaic layer 4 can be located anywhere within the structureforming system 2 and/or at any of its external edges or faces. They canalso be configured to reflect light by means of specular reflection,TIR, or scattering. It should also be understood that photovoltaic layer4 can be located anywhere between such surfaces so as to provide for themultiple passage of the trapped light through the photoresponsivematerial of the photovoltaic layer 4 and promote absorption.

The present invention is not limited to the case when focusing array 6and photovoltaic layer 4 are disposed in a stationary position withrespect to each other and can also be applied to the case when focusingarray 6 and photovoltaic layer 4 can be disposed in any one of atranslated, a reversed and/or a rotated orientation relative to eachother in order, for example, to adjust the optical paths of the incidentrays or to track the source of light. Furthermore, focusing array 6 andphotovoltaic layer 4 can be made movable with respect to each other toadjust the positions of light input ports 14 with respect of the focalareas of respective lenses 10. Furthermore, a tracking device can beprovided to facilitate pointing of system 2 to the light source. Singleor multiple instances of system 2 can be provided in the form of panelsor plates and installed on an individual tracking unit. When system 2 isused to collect sunlight and convert it to electricity, the trackingdevice may include mechanical, hydraulic, electric and electroniccomponents such as are those well-known in the art. The tracking devicecan be implemented as a single-axis or two-axis tracker to follow thediurnal motion of the sun and maintain the prevailing plane of system 2generally perpendicular to the direction to the sun at least in oneangular dimension.

It will be appreciated that, in accordance with preferred embodiments,lenses 10 are provided for pre-focusing of at least the direct beamcomponent of the incident light into a number of smaller beams andinjecting said beams into photovoltaic layer 4 through a plurality ofsmall designated areas rather than through the entire surface area.Light input ports 14 are provided to facilitate the injection of thefocused beams and can also be located anywhere in system 2 provided theyare located in the vicinity of the focal areas of the respective lenses10 and are optically coupled to photovoltaic layer 4. Various examplesand configurations of focusing array 6 and light input ports 14 havebeen described and provided by way of examples and not limitations.Further examples and configurations will become apparent to thoseskilled in the art without departing from the scope and spirit of thisinvention.

Further details of operation of system 2 shown in the drawing figures aswell as its possible variations will be apparent from the foregoingdescription of preferred embodiments. Although the description abovecontains many details, these should not be construed as limiting thescope of the invention but as merely providing illustrations of some ofthe presently preferred embodiments of this invention. Therefore, itwill be appreciated that the scope of the present invention fullyencompasses other embodiments which may become obvious to those skilledin the art, and that the scope of the present invention is accordinglyto be limited by nothing other than the appended claims, in whichreference to an element in the singular is not intended to mean “one andonly one” unless explicitly so stated, but rather “one or more.” Allstructural, chemical, and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the present invention, for it to be encompassedby the present claims. Furthermore, no element, component, or methodstep in the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. 112, sixth paragraph, unlessthe element is expressly recited using the phrase “means for.”

1. A light harvesting system, comprising: a photoresponsive layerconfigured to internally absorb at least a portion of the lightpropagating through its body; a plurality of light input portsassociated with a light receiving surface of said photoresponsive layer;and a plurality of light collecting elements within a planar focusingarray configured for focusing received light onto said light inputports; wherein each of said plurality of light input ports is configuredto communicate incident light into said photoresponsive layer at asufficiently oblique angle so as to increase the optical path of saidlight through said photoresponsive layer.
 2. A system as recited inclaim 1 further comprising at least one electrical contact associatedwith said photoresponsive layer.
 3. A system as recited in claim 1further comprising at least one electrical contact associated with saidphotoresponsive layer, wherein said electrical contact is made from anoptically transparent material.
 4. A system as recited in claim 1further comprising at least one electrical contact associated with saidphotoresponsive layer, wherein said electrical contact comprises areflective metallic material and is made in the form of a grid.
 5. Asystem as recited in claim 1 further comprising a plurality ofelectrical contact fingers associated with the light receiving surfaceof said photoresponsive layer and disposed in spaces between pairs ofadjacent said light input ports and away from the light paths of thefocused beams formed by said light collecting elements.
 6. A system asrecited in claim 1 further comprising at least one electrical contactassociated with said photoresponsive layer, wherein said electricalcontact comprises a reflective metallic material and is made in the formof a thin sheet or a film.
 7. A system as recited in claim 1, whereinsaid photoresponsive layer comprises at least one photovoltaic cell. 8.A system as recited in claim 1, wherein said planar focusing arraycomprises a lenticular lens array.
 9. A system as recited in claim 1,wherein said planar focusing array comprises a point-focus lens array.10. A system as recited in claim 1, wherein said focusing arraycomprises point focus lenses and wherein said point focus lenses have ashape selected from the group consisting of round, rectangular, square,and hexagonal.
 11. A system as recited in claim 1, wherein each of saidplurality of said light collecting elements is selected from the groupof optical elements consisting of imaging lenses, non-imaging lenses,spherical lenses, aspherical lenses, lens arrays, Fresnel lenses, TIRlenses, gradient index lenses, diffraction lenses, mirrors, Fresnelmirrors, spherical mirrors, parabolic mirrors, mirror arrays, and troughmirrors.
 12. A system as recited in claim 1, wherein each of saidplurality of light input ports is disposed in a predetermined alignmentwith each of said plurality of light collecting elements.
 13. A systemas recited in claim 1, wherein each said angle is so selected as toresult in the propagation of at least a substantial portion of lightrays at sufficiently high angles, above the predetermined critical anglefor total internal reflection (TIR), with respect to a surface normal toat least one surface of said photoresponsive layer.
 14. A system asrecited in claim 1, wherein each of said plurality of light input portscomprises a refractive face inclined at an angle with respect to theprevailing plane of said photoresponsive layer.
 15. A system as recitedin claim 1, wherein each of said plurality of light input portscomprises a reflective face inclined at an angle with respect to theprevailing plane of said photoresponsive layer.
 16. A system as recitedin claim 1, wherein said plurality of light input ports comprises aparallel array of elongated grooves.
 17. A system as recited in claim 1,wherein each of said plurality of light input ports comprises at leastone cavity.
 18. A system as recited in claim 1, wherein each of saidplurality of light input ports comprises at least one cavity having asufficiently high aspect ratio.
 19. A system as recited in claim 1,wherein each of said light input ports comprises at least one surfacerelief feature selected from the group of elements consisting ofcavities, holes, extensions, bulges, prisms, prismatic grooves, cones,conical cavities, funnel-shaped cavities, surface texture, reflectivesurfaces, refractive surfaces, diffraction gratings, holograms, andlight scattering elements.
 20. A system as recited in claim 1, furthercomprising an optical interface disposed between said photoresponsivelayer and said focusing array, wherein said optical interface ischaracterized by a drop in refractive index in the direction of lightpropagation from said photoresponsive layer toward said focusing array.21. A system as recited in claim 1, further comprising means forpromoting a total internal reflection in said photoresponsive layer. 22.A system as recited in claim 1, wherein said focusing array and saidphotoresponsive layer are adapted for being retained in a translated, areversed and/or a rotated orientation relative to each other towardadjusting the acceptance angle or for tracking the source of light. 23.A system as recited in claim 1 further comprising at least one trackingmeans for tracking a light source.
 24. A photovoltaic sunlightharvesting system having a layered structure, comprising: a photovoltaiclayer disposed between a first and a second reflective surfaces; a lightfocusing layer having surface relief features each configured to focusthe incident light; wherein at least one of said first and secondreflective surfaces comprises light input ports disposed in energyreceiving relationship to said surface relief features and configured toinject incident light into the space between said first and secondreflective surfaces so as to cause multiple passage of light throughsaid photovoltaic layer.
 25. A system as recited in claim 24, wherein atleast one of said first and second reflective surfaces is configured forreflecting light by means of at least TIR.
 26. A system as recited inclaim 24 further comprising at least one cladding layer associated withat least one of said first and second reflective surfaces.
 27. A systemas recited in claim 24, wherein at least one of said first and secondreflective surfaces is associated with a mirror layer comprising ametallic material.
 28. A system as recited in claim 24, wherein at leastone of said first and second reflective surfaces is associated with amirror layer comprising a Bragg reflector.
 29. A system as recited inclaim 24, wherein said light focusing layer comprises a lens array. 30.A system as recited in claim 24, wherein said photovoltaic layercomprises at least one photovoltaic cell.
 31. A system as recited inclaim 24, wherein said photovoltaic layer is associated with at leastone light detector.
 32. A system as recited in claim 24, wherein each ofsaid light input ports is formed by a microstructured area in at leastone of said first and second reflective surfaces.